PHP session variables problem in Firefox

Hi,
I have developed a small application for my company on PHP but have some small difficulty to overcome. In fact some session variables are getting vanished mysteriously.
Here is the flow :

index.php->page1.php->page2.php->page3.php

Here are the excerpts from the files.

index.php :

session_start();
session_register("username_session");
session_register("fullname_session");
$_SESSION["username_session"]=<value from database>
$_SESSION["fullname_session"]=<value from database>

page1.php :

Nothing related to session or session variables in this page. Just pass on a few form fields to next page.
The form fields are "firstvar" and "secondvar"

page2.php :

session_start();
session_register("firstvar_session");
session_register("secondvar_session");

$_SESSION["firstvar_session"]=$_POST["firstvar"]
$_SESSION["secondvar_session"]=$_POST["secondar"]


page3.php:

I access session variables here

session_start();
echo "username=".$_SESSION["username_session"]."<BR>";
echo "fullname=".$_SESSION["fullname_session"]."<BR>";
echo "firstvar=".$_SESSION["firstvar_session"]."<BR>";
echo "secondvar=".$_SESSION["secondvar_session"]."<BR>";



Here is the output :

username=ganesh
fullname=M. Ganesh
firstvar=
secondvar=